Integrated circuits are often formed on substrates, such as substrates of semiconducting material. Such substrates can hold as few as one or many as thousands of the integrated circuits. As the term is used herein, “integrated circuit” includes devices such as those formed on monolithic semiconducting substrates, such as those formed of group IV materials like silicon or germanium, or group III-V compounds like gallium arsenide, or mixtures of such materials. The term includes all types of devices formed, such as memory and logic, and all designs of such devices, such as MOS and bipolar. The term also comprehends applications such as flat panel displays, solar cells, and charge coupled devices.
Integrated circuits are subjected to many different tests and analyses during the fabrication cycle, to determine whether the materials and structures of the integrated circuits are formed correctly. Such tests typically include ellipsometric or reflectometric analysis of the properties of various layers.
In general, any broad band ellipsometric measurement that extends into the deep ultraviolet or vacuum ultraviolet region is achieved by means of two or more light sources. Each light source provides a specific and limited wavelength range according to the nature and physics of the emission phenomenon, which is usually confined in an assembly such as a lamp. Two or more lamps are required—a separate lamp for each desired wavelength range—in order to obtain the desired signal to noise ratio in the measurement system. In a generic broad-band ellipsometer, the spectrum needed for measurement purposes is collected using all of the required lamps as light sources of the system. These lamps can be used individually, with each having its own specific optical path, or they can be combined in the same optical path. In order to perform a broad-band measurement, the system is engineered to collect measurement signals that come back from each light source in a serial mode, thus using the wavelength range of each lamp once per acquisition cycle. This is generally referred to as a combined measurement.
Specific applications, measurement sequences, or test conditions for ellipsometry may require a higher signal-to-noise ratio than is provided by the configurations generally described above. To achieve a higher signal to noise ratio it is often necessary to increase the integration time of the signal. For a rotating polarizer ellipsometer, this is referred to as an increased number of scans.
When a higher number of scans are required in a combined measurement, the system is used to independently and sequentially collect each wavelength portion of the signal, each with a specific number of scans, prior to moving to the next signal acquisition. The measurement result is provided as a combined regression of the multiple wavelength range signals.
Unfortunately, the deep ultraviolet and vacuum ultraviolet portions of electromagnetic spectrum are highly energetic, and thus interfere with the atomic or molecular structure of the sample. As a result, there is interaction between the measurement signal and the sample, both at the surface and also within the body of the sample. The main phenomenon at the surface is commonly understood to be the interaction of the light with contaminants at thermodynamic equilibrium. The interaction in the body is thought to be driven by atomic absorption of the light photons. As a result, combining vacuum ultraviolet measurements with higher numbers of scans causes higher magnitude effects, which condition is aggravated while performing repeated measurement acquisitions at the same physical location. This results in the measurements showing a trend due to the change resulting from repeated vacuum ultraviolet or deep ultraviolet exposure.
What is needed, therefore, is a system that overcomes problems such as those described above, at least in part.